Chemical polishing composition and method for aluminum metals



tax-ro United States Patent 3,041,227 CHEMICAL POLISHING COMPOSITION AND METHOD FOR ALUMINUM METALS John F. Jumer, Elmhurst, 11]. No Drawing. Filed Sept. 19, 1958, Ser. No. 761,955 12 Claims. (Cl. 156-21) My invention relates to the chemical polishing of metals and particularly to nonferrous metals such as aluminum.

In the past several years a number of bright dipping or chemical polishing bath compositions have been proposed and used in various localities throughout the United States. A number of these have been widely accepted due to their relatively low cost and ease of proper operation. Of these baths which have been accepted, those based on the use of phosphoric acid have been the most successful, particularly when they could properly be classed as chemical polishing rather than bright dipping baths. Chemical polishing is considered preferable to bright dipping since the former produces a surface leveling action, attacking the microscopic surface projections preferentially to the valleys, while a bright dipping bath tends to act uniformly over the surface and produces little or no leveling action.

Substantially all of the chemical polishing or bright dipping baths occasionally produce irregularly polished or pitted surfaces particularly on articles of aluminum or aluminum alloy. It has been noted that this unsatisfactory surface condition occurs when the gaseous film forming at the interface of the metal and bath is relatively thick or irregular and when the size of the bubbles discharged from the film are either irregular or large as contrasted to small and uniform in size. Another characteristic of the film usually indicative of unsatisfactory action is the formation of a more or less static fihn in which the bubbles formed tend to hang to the work surface rather than being promptly released. Control of the film characteristics however has proven particularly diflicult and has in some instances demanded complete revision of equipment and bath composition.

One of the principal objects of my invention is a chemical polishing bath for aluminum which contains an acid stable surface active agent capable of controlling the thickness, mobility and uniformity of the gaseous film which forms around the object being treated as well as insuring liberation of many small uniformly sized bubbles thereby yielding consistently a bright uniformly polished aluminum surface practically free from pits or unpolished areas.

Other objects include the provision of additive agents for chemical polishing baths for aluminum which permit a reduction in the bath temperature, which increase the tolerance of the bath toward undesirable compounds such as aluminum phosphate, which do not yield excessive quantities of fumes, which are relatively inexpensive and which are sufficiently nontoxic to permit handling without special precautions.

Other objects as well as the many advantages of my invention will be disclosed in the following detailed description of my invention.

Essentially, m ntion involves the addition to a bath consistingg rincipally of orthophosihorrc aci'g' E1 small quantity 0 a. uorma e sur ace active acid stable m acidic media and preferably a fiuorinated aliptggtic car oxy c aci en use in quantities varying from Mt to approximately 1 percent by weight, these compounds exert a uniform surface tension effect across the entire work surface which apparently aids materially in maintaining the desired film characteristics and in insuring substantially uniform activity on the exposed surface of the article being treated.

URUOO UEl'lElibC More specifically, a satisfactory bath for the chemical polishing of aluminum may be prepared by adding to 100 ccs. of commercial orthophosphoric acid (75%), about 6 grams Of gr ll-1mm and 1.5 grams of sodium 5 sulfate. To this I add about of 1 percent by volume trifluoroacetic acid (CF OOH). The temperature of the resulting bath may be maintained at 180 to 210 F. and the article immersed for from 5 to 180 seconds. It is of course not essential that 75 percent orthophosphoric l0 acid be employed since the 85 percent grade is also suitable as are similar compounds prepared from acid anhydrides.

Another suitable bath may be prepared by utilizing 94 percent by volume orthophosphoric acid mixed with 15 6 percent by volume nitric acid. In this instance the orthophosphoric acid should be of the commercial 85 percent grade and the nitric acid of the 36 Baum grade. To this bath, as in the previous example, I add from $5 0 of 1 percent to 1 percent by volume of a fiuorinated aliphatic carboxylic acid, such as trifluoroacetic acid.

For general use, trifluoroacetic acid is the best of the commonly available fiuorinated hydrocarbon agents for use as an additive in phosphoric acid baths intended for use in chemically polishing aluminum. Its boiling point (72.4 C.) is somewhat below the usual operating temperature of the bath which allows gradual evaporation and loss of the agent during normal bath utilization. As this loss is occurring, other bath components such as the nitrates are also being lost or rendered inelfective. Thus, when necessary the bath may be refreshed or restored to a satisfactory operating condition by adding predetermined quantities of the necessary materials such as trifiuoroacetic acid and nitrates at the same time without danger of unintentionally exceeding the optimum concentrations. This is considered desirable because the chemical determination of trifluoroacetic acid and its equivalent compounds is not a simple operation and excessive quantities can prove harmful.

Although trifluoroacetic acid constitutes a preferred material, not only because of its physical characteristics but also because of its relatively low cost and availability, there are nevertheless related compounds which yield satisfactory results when used in the quantities indicated and which have characteristics of particular use under certain circumstances. Monofluoroacetic acid (CH FCOOH) for example has a relative y g oiling W). Solubility of this compound in aqueous phosphoric acid is so limited that excessive concentrations cannot be created. Thus, this compound is quite useful in baths utilizing high operating temperatures even thoigh its activity is somewhat less than trifiuoroacetic ac:

Difluoroaeetic acid (CHFgCOOH) also has a relatively o g pom ut this compound has a solubility in aqueous phosphoric acid which makes possible the creation of an undesirable overconcentration of the compound in all but the very highest temperature baths. Thus, the use of this compound would be indicated either where the temperature of the bath exceeds about 140 C. or where the bath is periodically replaced in its entirety without the use of additives.

Orthofluoro benzoic acid constitutes an example of J the several uonnate cyc c car y c act s w 1c may beemoe. iyo .2 part for 100 parts of water and a melting point of 122 C. Although such compounds may be utilized, the fiuorinated aliphatic acids are preferred for general utilization.

The addition of these surface active agents such as trifluoroacetic acid to phosphoric acid aluminum polishing baths produces a number of desirable effects. In their presence the gaseous film which forms at the work CAHlYl I l IL surface becomes substantially uniform both in thickness and in density. The bubbles which form the film are more nearly uniform in size and are quite small. In

addition they are promptly released, forming what may be termed a highly mobile uniform film. Uniform brightly polished aluminum surfaces can be obtained with considerably less care than has heretofore been necessary. In addition, the presence of even the minimum amount of agent in the bath permits a reduction in the temperature of the bath, usually on the order of 10 F., without any apparent harmful effect on the quality of the work. This reduction in temperature is highly desirable since it reduces the rate at which fumes and vapors are generated and prolongs the life of the bath. Similarly, concentrations of deleterious compounds, such as aluminum phosphate, may be permitted to exceed the usual limits before taking steps to reduce concentration. It has also been noted that the amount of bath solution lost by drag-out is somewhat reduced.

From the foregoing it will be apparent that the use of the fiuorinated hydrocarbon additive agent is beneficial from many aspects. Over and above the marked improvement in finish and the reduction of rejected work pieces, the lower bath temperatures, the reduction in drag-out and the increased tolerance of the bath to aluminum phosphate effect economies of operation which more than offset the increased cost of the addition of fiuorinated hydrocarbon.

I claim:

1. A composition for the chemical polishing of aluminum and aluminum alloys consisting essentially of a solution of orthophosphoric acid having a concentration of H P not less than about 75 by weight and containing from about 0.001% to about 1.0% by weight of a member selected from the group consisting of a fiuorinated lower aliphatic carboxylic acid and a fiuorinated benzoic acid.

2. A composition for the chemical polishing of aluminum and aluminum alloys consisting essentially of a solution of orthophosphoric acid having a concentration of H PO not less than about 75% by weight and containing from about 0.001% to about 1.0% by weight of a fiuorinated acetic acid.

3. A composition for the chemical polishing of aluminum and aluminum alloys consisting essentially of a solution of orthophosphoric acid having a concentration of H PO not less than about 75 by weight and containing from about 0.001% to about 1.0% of trifluoroacetic acid, by weight.

tration of H PO not less than about 75% by weight and 4 containing from about 0.001% to about 1.0% by weight of a member selected from the group consisting of a fiuorinated lower aliphatic carboxylic acid and a fiuorinated benzoic acid.

8. A method for the chemical polishing of aluminum and aluminum alloys which comprises applying to the surface thereof at a temperature between about 180 F. and about 210 F. a composition consisting essentially of a solution of orthophosphoric acid having a concentration of H PO not less than about by weight and containing from about 0.001% to about 1.0% by weight of trifluoroacetic acid, for a period between about 5 and 180 seconds.

9. A composition for the chemical polishing of aluminum and aluminum alloys consisting essentially of a solution of orthophosphoric acid having a concentration of H PO not less than about 75 by weight and containing a minor proportion of nitric acid and from about 0.001% to about 1.0% by weight of a fiuorinated acetic acid.

10. A method for the chemical polishing of aluminum and aluminum alloys which comprises applying to the surface thereof at a temperature between about 180 F. and about 210 F. a composition consisting essentially of a solution of orthophosphoric acid having a concentration of H PO not less than about 75 by weight and containing a minor proportion of nitric acid and from about 0.001% to about 1.0% by weight of a member selected from the group consisting of a fiuorinated lower aliphatic carboxylic acid and fiuorinated benzoic acid.

11. A composition for the chemical polishing of aluminum and aluminum alloys consisting essentially of a solution of orthophosphoric, acid having a concentration of H PO not less than about 75 by weight and containing a minor proportion of nitric acid and from the group consisting of a fiuorinated lower aliphatic carboxylic acid and a fiuorinated benzoic acid.

12. A composition for the chemical polishing of aluminum and aluminum alloys consisting primarily of orthophosphoric acid, a minor quantity of nitric acid, water and from about 0.001% to about 1.0% by weight of a memlber selected from a group consisting of a fiuorinated aliphatic carboxylic acid and a fiuorinated benzoic acid dissolved therein.

References Cited in the file of this patent UNITED STATES PATENTS 1,525,606 Gravel] Feb. 10, 1925 2,257,960 Humphrey et a1 Oct. 7, 1941 2,516,685 Douty et al. July 25, 1950 2,652,360 Bond et al. Sept. 15, 1953 2,692,189 Bo-Shin Ro Oct. 19, 1954 2,729,551 Cohn Jan. 3, 1956 2,788,362 Bamhart et al. Apr. 9, 1957 OTHER REFERENCES Harris: Trend in Aluminum Cleaning, pp. 28-32, Aluminum and Magnesium Magazine, April 1945.

Schwartz et al.: Surface Active Agents and Detergents; vol. II; copyright I an. 23, 1958, by Interscience Publishers, Inc. New York; pp. -152. 

7. A METHOD FOR THE CHEMICAL POLISHING OF ALUMINUM AND ALUMINUM ALLOYS WHICH COMPRISES APPLYING TO THE SURFACE THEREOF AT A TEMPERATURE BETWEEN ABOUT 180*F. AND ABOUT 210*F. A COMPOSITION CONSISTING ESSENTIALLY OF A SOLUTION OF ORTHOPHOSPHORIC ACID HAVING A CONCENTRATION OF H3PO4 NOT LESS THAN ABOUT 75% BY WEIGHT AND CONTAINING FROM ABOUT 0.001% TO ABOUT 1.0% BY WEIGHT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF A FLUORINATED LOWER ALIPHATIC CARBOXYLIC ACID AND FLUORINATED BENZOIC ACID. 